1. Field of the Invention
The present invention relates to a driving device configured to rotate a rotor yoke, and a light amount controller incorporating the driving device as a drive source thereof.
2. Description of the Related Art
Conventionally, there have been proposed various types of small-sized and slim driving devices (see e.g. Japanese Laid-Open Patent Publications (Kokai) No. 2002-272082 and No. 2002-049076). Now, FIGS. 8 and 9 show a conventional driving device disclosed in Japanese Laid-Open Patent Publications (Kokai) No. 2002-272082.
FIG. 8 is an exploded perspective view of the conventional driving device. FIG. 9 is a cross-sectional view in the axial direction of the driving device in an assembled state thereof.
As shown in FIGS. 8 and 9, the driving device is comprised of a magnet 101, a coil 102, a stator 104, a base plate 105, a magnet presser 106, shutter blades 107, and a cover 109.
The magnet 101 is in the form of a hollow cylinder, is configured to pivotally move, and has an outer peripheral surface circumferentially divided into n sections magnetized to have alternately different poles. The magnet 101 is formed with a rotation restricting pin 101a and output pins 101b. The coil 102 is disposed coaxially with the magnet 101. The stator 104 is formed of a soft magnetic material, for being magnetized by the coil 102, and includes comb tooth-shaped outer magnetic pole portions 104a and a hollow cylindrical inner magnetic pole portion 104b. The outer magnetic pole portions 104a of the stator 104 are opposed to the outer peripheral surface of the magnet 101, while the inner magnetic pole portion 104b of the stator 104 is opposed to the inner peripheral surface of the magnet 101.
In the driving device, the polarity of the outer magnetic pole portions 104a and that of the inner magnetic pole portion 104b are switched by changing the direction of energization of the coil 102, whereby the magnet 101 is angularly reciprocated within a limited range.
In the driving device configured as above, magnetic flux generated by energization of the coil 102 flows from each of the outer magnetic pole portions 104a to the opposed inner magnetic pole portion 104b, or reversely flows from the inner magnetic pole portion 104b to each of the outer magnetic pole portions 104a, to effectively act on the magnet 101 located between the outer magnetic pole portions 104a and the inner magnetic pole portion 104b. The distance between the outer magnetic pole portions 104a and the inner magnetic pole portion 104b can be set to the sum of the thickness of the magnet 101, the clearance between the magnet 101 and each outer magnetic pole portion 104a, and the clearance between the magnet 101 and the inner magnetic pole portion 104b, which makes it possible to reduce the resistance of a magnetic circuit formed by each outer magnetic pole portion 104a and the inner magnetic pole portion 104b. As a result, a large amount of magnetic flux can be generated by a small amount of electric current, which makes it possible to enhance the efficiency of the driving device.
There is an increasing demand for utilization of the above-described conventional driving device as a drive source of a light amount controller, such as a shutter mechanism or a diaphragm mechanism in cameras, for achieving high-speed driving of the shutter mechanism or the diaphragm mechanism. However, since the conventional driving device employs the hollow cylindrical magnet as the rotor, there is a limit to reduction of the moment of inertia, which makes it difficult to increase the driving speed of the above-mentioned mechanism.
Another problem is that although the torque of the driving device can be enhanced by increasing the thickness of the magnet and thereby obtaining a stronger magnetic force, the increased thickness of the magnet leads to an increase in the moment of inertia of the rotor, which makes it impossible to achieve high-speed driving of the above-mentioned mechanism.
Further, in the conventional driving device, the number of component parts is reduced by forming the rotation restricting pin 101a and the output pins 101b of the magnet 101 out of a plastic magnetic material, but the plastic magnetic material is more fragile than a metal or a resin, so that the pins are often broken by a physical shock. To prevent this, it is necessary to form the pins using a material other than the plastic magnetic material. However, when another material is used, reduction of the number of component parts and enhancement of rigidity of the output pins become incompatible.